Siedzieliśmy w kawiarni, aż ulica zrobiła się prawie pusta.

Siedzieliśmy is the past tense, 1st person plural of siedzieć (to sit).

So it breaks down like this:

• siedzieć = to sit
• siedzieliśmy = we were sitting / we sat

The ending -liśmy tells you it is we in the past.

Because siedzieć is imperfective, this form often means an ongoing situation in the past:

• Siedzieliśmy w kawiarni = We were sitting in a café

That fits the sentence well, because it describes a continuing action that lasted up to a certain point.

After w when it means in a place, Polish normally uses the locative case.

So:

• kawiarnia = café
• w kawiarni = in the café

This is a very common pattern:

• w domu = in the house
• w szkole = at school / in the school
• w restauracji = in the restaurant

So w kawiarni is exactly what you expect after w for location.

Here aż means until, or more naturally in English, all the way until.

The idea is:

• Siedzieliśmy w kawiarni, aż...
• We stayed/sat in the café until...

It often suggests that something continued up to a later moment or result.

So the sentence gives this feeling:

• we remained there
• time passed
• then the street became almost empty

In this sentence, aż connects the first situation with the point it lasted until.

Because aż introduces a subordinate clause here, and Polish normally puts a comma before such clauses.

So the structure is:

• Siedzieliśmy w kawiarni,
• aż ulica zrobiła się prawie pusta.

That comma is standard Polish punctuation.

Literally, zrobiła się looks like made itself, but in real Polish it very often means became.

So:

• zrobić się = to become / to get

Examples:

• Zrobiło się ciemno. = It got dark.
• Zrobiło się zimno. = It got cold.
• Ulica zrobiła się pusta. = The street became empty.

This is a very common way to describe a change of state.

Because it agrees with ulica, and ulica is a feminine singular noun.

So in the past tense:

• masculine singular: zrobił się
• feminine singular: zrobiła się
• neuter singular: zrobiło się

Since the subject is ulica (street), you need:

• ulica zrobiła się...

Pusta is the adjective empty in the feminine singular nominative form, agreeing with ulica.

Agreement in Polish means the adjective must match the noun in:

• gender
• number
• case

So:

• pusty = masculine
• pusta = feminine
• puste = neuter

Because ulica is feminine, you get:

• pusta ulica = an empty street
• ulica zrobiła się pusta = the street became empty

Prawie means almost.

It modifies pusta, so:

• prawie pusta = almost empty

This means the street was not completely empty, but very close to it.

You can use prawie the same way with many other words:

• prawie gotowy = almost ready
• prawie koniec = almost the end
• prawie nigdy = almost never

Polish is perfectly happy to make the street the subject of a change-of-state verb:

• ulica zrobiła się prawie pusta

That literally follows the pattern:

• the street became almost empty

This is natural in Polish. The street is treated as the thing whose condition changed.

English may sometimes prefer:

• the street was almost empty or
• the street became almost empty

Polish can use either a static description or a change-of-state structure depending on what it wants to emphasize. Here the sentence emphasizes that the street gradually got emptier over time.

Zrobić się is a perfective verb. Perfective verbs focus on the change as a completed event or result.

So:

• siedzieliśmy = ongoing background action
• ulica zrobiła się prawie pusta = the point/result that was reached

That combination is very common in Polish:

• an imperfective verb for the ongoing situation
• a perfective verb for the event/result that happened during or at the end of it

So the sentence feels like:

• We were sitting in the café
• until the street had become almost empty

Yes. Depending on context, siedzieliśmy can often be rendered in English as:

• we were sitting
• we sat
• we stayed

In this sentence, English stayed is often very natural, because the point is not really their posture but the fact that they remained there for some time.

So all of these can work depending on style:

• We were sitting in the café until the street became almost empty.
• We stayed in the café until the street was almost empty.
• We stayed at the café until the street became almost empty.

Yes, but the meaning shifts a little.

• ulica była prawie pusta = the street was almost empty

  • this simply describes a state

• ulica zrobiła się prawie pusta = the street became almost empty

  • this emphasizes a change over time

In your sentence, zrobiła się works well because it matches the idea of waiting there until things changed outside.

Yes, you might also hear:

• ulica prawie opustoszała

The verb opustoszeć means to become empty / deserted.

So:

• Siedzieliśmy w kawiarni, aż ulica prawie opustoszała.

That is also natural, though it sounds a bit stronger and slightly more literary or expressive in some contexts.

The original sentence with zrobiła się prawie pusta is very clear and everyday.

Sometimes, but not always with exactly the same feel.

• aż = until, up to the moment when
• dopóki = as long as / until

A possible version would be:

• Siedzieliśmy w kawiarni, dopóki ulica nie zrobiła się prawie pusta.

Notice the nie after dopóki. In this kind of sentence, Polish often uses dopóki ... nie where English just says until.

Still, aż is very natural in your original sentence and often feels a bit more direct for a single endpoint.

It has two parts:

1. Siedzieliśmy w kawiarni
   = the ongoing background action

2. aż ulica zrobiła się prawie pusta
   = the endpoint/result

So the pattern is:

• We were/stayed X, until Y happened

More literally:

• We were sitting in the café, until the street became almost empty.

This is a very useful Polish pattern for describing something that continued up to a particular moment.